1. Field of the Invention
The present invention relates to a moistening device which moistens an image recording sheet (to be referred to as a sheet hereinafter) and an electrophotographic image printing apparatus which includes the moistening device and, more particularly, to a moistening technique of supplying moisture to a sheet to which an image has been fixed by a thermal fixing device.
2. Description of the Prior Art
In an image printing apparatus which prints an image by electrophotography, generally, a toner image is formed on a sheet with a fine toner powder. The toner image is fixed on the sheet by heating and pressurizing processes.
During the heating process in a fixing step, moisture evaporates from the sheet. As the sheet is exposed to the atmosphere, the sheet absorbs moisture, and the moisture to be contained in the sheet is recovered gradually. Moisture recovery is not uniform, however, and progresses with different proportions at different portions of the sheet. For example, when a plurality of sheets are left in a bundled state, moisture absorption of the sheets progresses fast in the peripheral portion of the sheet, but is slow or does not occur at the central portion of the sheet.
Due to a difference in moisture content, the elongation of the sheet differs depending on the location to cause a waving phenomenon on the sheet where an image has been printed.
The waving phenomenon occurs often in sheets which are printed and deposited on a delivery tray or in sheets deposited on a binding stacker, and is conspicuous particularly in sheets which are deposited in a large amount on a delivery tray including a large-volume stacker. Also, sometimes a sheet curls to form a curl.
According to a sheet moisture replacement system proposed in U.S. Pat. No. 5,264,899 (patent reference 1), a porous moistening roller is used to convey a sheet. Water is supplied to the moistening roller from its shaft core to moisten the sheet, thus preventing waving or the like of the sheet.
In this manner, deformation of a sheet after image printing includes waving deformation caused by a waving phenomenon which occurs because the elongation of the sheet differs depending on the location and deformation caused by a curling phenomenon which occurs as the sheet curls in the convey direction.
The waving phenomenon can be solved by supplying moisture to the sheet so that the entire surface of the sheet is moistened substantially uniformly. The curling phenomenon can be solved by making the moisture difference of obverse and reverse surfaces of the sheet small.
In a moistening device, a dampening mechanism damps one of two moistening rollers. The moistening roller which has been damped dampens the other moistening roller. A nip portion formed by the two moistening rollers moistens the sheet. With this moistening device, the moistening roller which is directly damped by the dampening mechanism dampens the sheet with a larger amount of water than in damping from the other moistening roller to the sheet. Therefore, the moisture difference of obverse and reverse surfaces of the sheet becomes large, and the curl phenomenon is easy to come to occur.